1. Field of the Invention
This invention relates to an optical displacement measurement device in which phase change of light produced by movement of a diffraction grating is detected to measure the amount of movement of the diffraction grating.
2. Description of Related Art
As an optical displacement measurement device for detecting the position of relative movement of a movable portion of, for example, a machine tool or a semiconductor manufacturing device, there is known such device exploiting a grating interferometer. This device is configured to detect the displacement of the position of a diffraction grating, recorded on a moving scale, by taking advantage of light interference.
The present inventors already proposed an optical displacement measurement device which, by superimposing two diffracted light beams, which are to interfere with each other, to high accuracy, is capable of detecting the position of movement of a moving portion to high resolution and to high accuracy. See Patent Publication 1, for example.
In the technique disclosed in this Patent Publication 1, a coherent light beam forms an optical image on a grating surface of a diffraction grating by a first image-forming means. A first diffracted light beam is collimated by a second image-forming means and illuminated on a reflector of a reflection optical system in a perpendicular direction at all times. With such configuration, even in case the optical axis of the first diffracted light beam has become deviated, the first diffracted light beam as reflected necessarily retraces the optical path it followed as the incident light, with there being no change in the optical image forming position on the grating surface of the diffraction grating. On the other hand, the optical axis of a second diffracted light beam, generated on diffraction of the first diffracted light beam, is not deviated, while there is no change in the optical path length. In this manner, there is no deterioration in the interference signal being detected even in case the diffraction grating has moved in a direction other than the direction parallel to the grating vector, for example, in case the diffraction grating has become tilted or distorted.
If, in an optical scale for a grating interferometer, detecting changes in the phase of light caused by movement of the diffraction grating to detect the position, the diffraction grating is exposed to outside, fingerprints or impurities tend to be affixed to the scale, or the scale may be grazed. Thus, in Patent Publication 2, for example, a transparent protective layer is formed on the surface of the diffraction grating to cover and protect it from contamination or grazing.
Among the position detectors in which light diffracted from a diffraction grating is made to interfere to detect the amount of movement of the diffraction grating from changes in the interference signal, there is such a detector that uses multi-mode semiconductor laser as a light source. With this light source, the length of coherence of the light source is limited to suppress the effect of wavelength variations otherwise brought about by changes in temperature. See Patent Publication 2, for example.
With the use of the arrangements disclosed in the Patent Publications 2 and 3, it is possible to implement an optical displacement measurement device which is strong against grazing or contamination of the diffraction grating or against wavelength variations otherwise brought about by changes in temperature. See Patent Publication 3, for example.